SCR has been proven to be highly effective at NOx reduction, and SCR units can generally be scaled to the size required. However, SCR units typically require the use of ammonia as a reducing reagent, and it is a common problem that ammonia is difficult and dangerous to store, especially in populated areas. Thus, the use of urea and ammonia generators such as described in U.S. Pat. No. 7,090,810 to Sun, et al., are often effective, but these devices can be troubled by issues related to injector bearding and deposits caused by conditions at the injector, incorrect installation, and nozzle damage.
Injector bearding can be caused when small droplets of aqueous urea drift upwards, carried by small scale local flow recirculation near the nozzle. They can then contact the surface of the injector and, over time, form deposits that often give a beard-like appearance. This type of injector deposit will not usually grow large enough to create large deposits elsewhere in the chamber. They become localized to near the nozzle, sometimes forming a thin crust on the underside of the injector body near the nozzle, or a conical ‘bugle’ or ‘beehive’ shape just behind the nozzle. However, there have been situations where the deposits on the injectors continued to grow.
Injector beards that continue to grow can interfere with the injector spray, diverting some droplets and causing them to combine with others into very large droplets. Dripping solution can then cause deposits to form on the chamber walls or exit pipe. The deposits could continue to grow until the chamber would no longer function.
When urea gasification for SCR or other like commercial units is needed, effective operation without fouling of equipment requires uniform distribution and rapid pyrolysis and/or hydrolysis. A proper velocity distribution of hot air before and after introduction of urea is critical for the operation of such a gasification chamber. While the concept of a perforated plate has been suggested to provide uniform flow prior to urea injection to provide a desirable gas pattern for urea distribution, in practice these devices have a limited capacity and, if not specially modeled and constructed, can cause improper reagent back flow or recirculation which can result in solid urea encrusting on the plate, chamber walls or near the nozzle, causing fouling and related problems. It would be desirable to avoid fouling, especially on the nozzle.
There is a particular need for such an apparatus which can convert urea to gaseous form without equipment fouling or excessive reagent usage or loss of pollution control effectiveness.